172 PRACTICAL ORGANIC CHEMISTRY weight of substance) should be added and the liquid heated for a moment until a clear solution is obtained and then allowed to cool. On prolonged boiling it decomposes. For the preparation of aminoazobenzene the dry powder is sufficiently pure. Yield, nearly theoretical. (C6H5NH9)oH2SO4 + 2NaNO2 + 2H2SO4 = 2C0H5N2.SO4H + Na2SO4 + 4H2O. CGH5N2.S04H + CCH5NH2 = C8H6N:N.NHC6H5 + H2SO4. N.B.—The sulphuric acid, set free in the second phase of the reaction, acts upon the sodium nitrite, so that one molecule only is required. Properties.—Golden yellow plates (from alcohol) m. p. 98° ; insoluble in water; it explodes when heated above its melting" point. Reaction.—Dissolve a little of the substance in alcohol and add a drop or two of an alcoholic solution of silver nitrate. A red crystalline precipitate of C6H6N:N.NAg.C6H5 is deposited. See Appendix, p. 285. PREPARATION 70. Aminoazobenzene (Aniline yellow), C0H5N:NC0H4NI-l2- Mene,/£/m^., 1861, 496; Kekule', Zeitsch.f. CA., 1866, 2, 689 ; Staedel, Bauer, Ber., 1886, 19, 1953. 10 grms. diazoaminobenzene. 25 „ aniline. 5 ,, aniline hydro-chloride. The finely powdered diazoaminobenzene, aniline hydro- chloride (see p. 156), and aniline are mixed together and heated to 40° for an hour. The mixture forms a clear, deep red solution. After standing for 24 hours at the ordinary temperature, the diazoaminobenzene is converted into aminoazobenzene. A, slight excess of moderately strong hydrochloric acid is added, care being taken that no great evolution of heat occurs. On cooling, the aminoazobenzene separates out together with aniline hydrochloride. It is filtered and washed with cold, very dilute hydrochloric acid, when small violet crystals of aminoazo- benzene hydrochloride remain on the filter. In order to obtain, the free base, the hydrochloride is warmed with dilute ammonia.